Health data from Galaxy Watch synced with iOS?

You need to download the Samsung Health App and pair the watch to it. The Samsung Health App can then share the watch data to Apple's own Health App.

https://apps.apple.com/us/app/samsung-health/id1224541484

Go to Settings->Health (the Apple one)->Data Access & Devices, and the App should appear their under Apps as a potential source.

You can turn on the options you want to Sync. The data will be synced between both Apps.

Yes, that is the only data the Samsung App makes available to Health.

You would need to contact Samsung and request more data be made available to Sync with the Health App. It's up them what data they allow to be synced.